Although a number of retrospective cohort studies suggest that blood lead levels presently considered "normal" have adverse impact on neuropsychological development, this issue continues to inspire debate. Some of the uncertainty issues from the fact that retrospective studies cannot distinguish among competing causal models of lead's role because they do not adequately document a child's lead exposure history. The proposed research program addresses this deficiency. It consists of four studies of two cohorts selected from a large population of infants for whom umbilical cord blood lead concentration was measured. The longitudinal cohort is a group of 249 infants whose developmental status and blood lead levels were measured at ages 6, 12, 18, and 24 months. At age 4 1/2, their neuropsychological status will be assessed (focusing on attentional, linguistic, and fine motor function), as well as current levels of blood lead and cadmium, free erythrocyte protoporphyrin, and serum ferritin. Data will be gathered on five classes of potential confounders: child, parent, family, socialization values and behavior, and extra-family contacts and supports. The neuropsychological status and blood lead levels of these children will be re-examined when they reach 8 years of age. Their success in school will also be assessed (teachers' ratings, incidence of grade retention, use of special educational services, and performance on standardized tests). Shed deciduous teeth will be collected and dentine lead levels examined in relation to both current neuropsychologic status and the comprehensive blood lead history available for each child (7 measurements spanning birth to 8 years). Specific issues to be addressed by these follow-up studies include: (1) sensitive or critical periods with regard to lead toxicity, (2) the cognitive functions most vulnerable, and (3) the environmental factors which increase or reduce a child's risk of lead-induced deficit. Contact has been maintained by mail with a second cohort of 3,661 children (the mail follow-up cohort). Shed deciduous teeth will be collected in order to estimate their total lead exposure. At age 8, their school performance will be assessed by means of teachers' ratings of classroom behavior, the incidence of grade retention, referral for special educational services, and performance on standardized tests. A fourth study, to be carried out on the longitudinal cohort, is an econometric study of the health costs of low-level lead exposure in children.